NCEA Level 3 Chemistry - Core Content and Exam Technique
This section covers NCEA Level 3 Chemistry, examined through four external achievement standards - thermochemical principles and properties of particles and substances (91390), organic compounds (91391), equilibrium principles in aqueous systems (91392) and oxidation–reduction processes (91393) - with spectroscopic methods (91388) assessed internally. Marks come from correct calculations, accepted notation, and explanations that link particle-level behaviour to observable properties.
Core rules and concepts
- Thermochemistry: ΔrH° = ΣΔfH°(products) − ΣΔfH°(reactants) using standard states, or Σ(bonds broken) − Σ(bonds formed). Spontaneity follows ΔG° = ΔH° − TΔS°, and ΔS° rises when gas moles increase or order decreases.
- Equilibrium: Kc changes only with temperature; compare Q with K to find the shift direction. Kw = 1.0 × 10⁻¹⁴ at 25 °C and pH = −log₁₀[H₃O⁺]. A buffer is a weak acid or base with its conjugate; pH = pKa + log([A⁻]/[HA]).
- Redox: balance half-equations in order - atoms, then O with H₂O, then H with H⁺, then charge with e⁻. E°cell = E°red(cathode) − E°red(anode); a positive value means the reaction is spontaneous under standard conditions.
- Organic: name by IUPAC, map reagents to products (LiAlH₄ reduces aldehydes and ketones; acidified KMnO₄ oxidises primary alcohols to carboxylic acids), and classify structural, geometric (cis/trans) and optical isomerism.
Worked example
Calculate the pH of 0.100 mol L⁻¹ ethanoic acid (Ka = 1.74 × 10⁻⁵). Ka = [H₃O⁺][CH₃COO⁻]/[CH₃COOH]. Let x = [H₃O⁺] = [CH₃COO⁻], so 1.74 × 10⁻⁵ = x²/(0.100 − x). Assume x ≪ 0.100: x² = 1.74 × 10⁻⁶, x = 1.32 × 10⁻³ mol L⁻¹. Since 1.3% ionises, the assumption holds. pH = −log₁₀(1.32 × 10⁻³) = 2.879, quoted to three decimal places to match three significant figures.
Traps that cost marks
- Ignoring dilution when solutions are mixed: moles are conserved, concentration is not.
- Putting pure solids, pure liquids or water into a Kc or Ka expression.
- Sign slips: exothermic means negative ΔH; the bonds-broken term is positive.
- Claiming a weak-acid approximation without justifying it (ionisation under about 5%).
- Mixing up oxidation (electron loss, oxidation number increases) with reduction.
- Drawing isomers that are identical molecules, or missing the chiral carbon that gives optical isomers.